Intervertebral or spinal discs, lie between adjacent vertebrae which are the interlocking bones of the spine that are stacked on top of one another. Each disc forms a fibrocartilaginous joint to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together. Individual discs allow very limited vertebral motion such as extension and flexion; however, considerable motion is possible when several discs combine forces. In addition to providing the shock absorbing function, these discs allow the entire vertebrae or portions thereof to flex, bend, and twist.
In humans, there is one disc between each pair of vertebrae, except for the first cervical segment, the atlas. The atlas is a ring around the roughly cone-shaped extension of the axis or second cervical segment. The axis acts as a post around which the atlas can rotate, allowing the neck to swivel. There are a total of twenty-three discs in the human spine, which are most commonly identified by specifying the particular vertebrae they separate. For example, the disc between the fifth and sixth cervical vertebrae is designated “C5-C6”, the disc between the fourth and fifth lumbar vertebrae would be “L4-L5” and so on.
Spinal discs consist of an outer annulus fibrosus, which surrounds the inner nucleus pulposus. The annulus fibrosus consists of several layers of fibrocartilage. The strong annular fibers of the capsule contain the nucleus pulposus and distribute pressure evenly across the disc. The nucleus pulposus contains loose fibers suspended in a mucoprotein gel having the consistency of jelly. The nucleus has a high water content (about 80-85%), with the remainder made up mostly of proteoglycan, type II collagen fibers and elastin fibers. The proteoglycan functions to trap and hold the water, which is what gives the nucleus its strength and resiliency.
The nucleus pulposus acts as a shock absorber, absorbing the impact of the body's daily activities and keeping the adjacent vertebrae separated, thus serving to protect the vertebrae, brain, and other structures such as the nerves extending from the spinal column. By analogy, the disc can be likened to a filled doughnut; the annulus fibrosis is similar to the dough and the nucleus pulposus is the jelly. If, for example, one presses down on the front of the doughnut the jelly moves posteriorly or to the back and vice versa. In the condition of a prolapsed disc, the jelly/nucleus pulposus is forced out of the doughnut/disc. In the partially deflated state, the adjacent vertebrae may put pressure on the spinal nerves located near the disc, causing chronic back pain, sciatica, numbness, weakness and other syndromes which often require surgical intervention.
Spinal discs can crack or rupture through aging, injury, and illness. For instance, as people age, the nucleus pulposus begins to dehydrate, which limits its ability to absorb shock. The annulus fibrosus gets weaker with age and begins to tear. While this may not cause pain in some people, in others one or both of these may cause chronic pain.
Spinal discs are structures that are, by and large, prone to degenerative changes associated with wear and tear aging, misuse (e.g. smoking) and repeated trauma, such as frequent heavy lifting as well as genetic predisposition. These degenerative changes can cause the loss of normal structure and/or function. Over time the collagen or protein structure of the annulus fibrosus weakens and may become structurally unsound. Additionally, both water and proteoglycans (i.e. molecules that attract water content) decreases. Age-related changes that cause degenerative disc disease include a loss of fluid in the discs and tiny tears or cracks in the annulus or capsule of the disc. A sudden or acute injury leading to a herniated disc may also begin the degeneration process. Stress from motion may also result in a disc problem (e.g. herniation). These changes are linked and lead to the disc's inability to handle mechanical stress and the breakdown of the discs can result in back or neck pain, weakness, and loss of function as well as osteoarthritis, herniated disc, or spinal stenosis.
One generally refers to the gradual dehydration of the nucleus pulposus as degenerative disc disease. Degenerative disc disease is somewhat of a misnomer in that it is not really a disease, but rather a degenerative cascade that at times can produce pain from a damaged disc. Degenerative disc disease can take place throughout the spine, but it most often occurs in the discs in the lumbar region and the cervical region. Herniation of a spinal disc and the often resultant symptoms of intractable pain, weakness, sensory loss, incontinence and progressive arthritis are among the most common of debilitating afflictions associated with this process. As used herein, degenerative disc disease will be understood to refer to all chronic or acute states of physical and physiological changes of the spinal discs resulting in pain or discomfort.
When the annulus fibrosus tears due to an injury or the aging process, the nucleus pulposus can begin to extrude through the tear. This is called disc herniation. Near the posterior side of each disc, all along the spine, major spinal nerves extend out to different organs, tissues, extremities etc. It is very common for the herniated disc to press against these nerves, commonly known as a pinched nerve, causing radiating pain, numbness, tingling, and diminished strength and/or range of motion.
In addition, the contact of the inner nuclear gel, which contains inflammatory proteins, with a nerve can also cause significant pain referred to as radicular pain or radiculitis. A common form of radiculitis is sciatica, a radicular pain that radiates along the sciatic nerve from the lower spine to the lower back, gluteal muscles, back of the upper thigh, calf, and foot as often secondary to nerve root irritation from a spinal disc herniation or from osteophytes in the lumbar region of the spine. Pain due to the inability of the dehydrating nucleus pulposus to absorb shock is called axial pain or disc space pain.
Herniated discs go by many names and these can mean different things to different medical professionals. A slipped disc, ruptured disc, or a bulging disc can all refer to the same medical condition.
Typically, patients suffering from degenerative disc disease, including disc herniation, may be treated conservatively through a regimen of pain medications, physical therapy, exercise, immobilization, acupuncture and combinations of the foregoing. In the case of a herniated disc, if a patient's condition does not improve after conservative treatment, and if there is clear physical evidence of nerve root or spinal cord compression apparent and confirmed through radiological means, surgical removal of the herniated disc may be indicated. The process of discectomy—as the name implies—involves the simple removal of the disc without attempt to replace or repair the malfunctioning unit.
Statistics suggest that surgical techniques such as discectomies, are likely to result in short-term relief, but will not prevent the progressive deterioration of the patient's condition in the long run. Through better pre-operative procedures and diagnostic studies, long-term patient results have improved somewhat. But it has become clear that unless the removed disc is replaced or the spine is otherwise properly supported, further degeneration of the patient's condition will almost certainly occur.
In the mid-1950's and 60's, Cloward and Smith & Robinson popularized anterior surgical approaches to the cervical spine for the treatment of cervical degenerative disc disease and related disorders of the vertebrae, spinal cord and nerve root; these surgeries involved disc removal followed by interbody fusion with a bone graft. It was noted by Robinson (Robinson, R. A.: The Results of Anterior Interbody Fusion of the Cervical Spine, J. Bone Joint Surg., 440A: 1569-1586, 1962) that after surgical fusion, osteophyte (bone spur) reabsorption at the fused segment might take place. However, it has become increasingly apparent that unfused vertebral segments at the levels above and below the fused segment degenerate at accelerated rates as a direct result of this fusion. This has led some surgeons to perform discectomy alone, without fusion, by a posterior approach in the neck of some patients. However, as has occurred in surgeries involving the lower back where discectomy without fusion is more common as the initial treatment for disc herniation syndromes, progressive degeneration at the level of disc excision is the rule rather than the exception. Premature degenerative disc disease at the level above and below the excised disc can and does occur.
Discectomy procedures have inherent risks since the portion of the disc to be removed is immediately adjacent the nerve root and any damage to the nerve root is clearly undesirable. Further, the long-term success of discectomy procedures is not always certain due to the loss of nucleus pulposus which can lead to a loss in disc height. Loss of disc height increases loading on the facet joints which can result in deterioration of the joint and lead to osteoarthritis and ultimately to foraminal stenosis, pinching the nerve root. Loss of disc height also increases the load on the annulus as well. As the annulus fibrosis has been shown to have limited healing capacity subsequent to discectomy. A compromised annulus may lead to accelerated disc degeneration which may require spinal interbody fusion or total disc replacement.
Spine surgery occasionally involves fusion of the spine segments. In addition to the problems created by disc herniation, traumatic, malignant, infectious and degenerative syndromes of the spine can be treated by fusion. Other procedures can include bone grafts and heavy duty metallic rods, hooks, plates and screws being appended to the patient's anatomy; often they are rigidly and internally fixed. None provide for a patient's return to near-normal functioning. Though these procedures may solve a short-term problem, they can cause other, longer term, problems.
A number of attempts have been made to solve some of the problems described above by providing a patient with spinal disc prostheses, or artificial discs of one sort or another.
For example, Steffee, U.S. Pat. No. 5,071,437, describes a spinal disc prosthesis having upper and lower rigid flat plates and a flat elastomeric core sandwiched between the plates. Frey et al., U.S. Pat. Nos. 4,917,704 and 4,955,908, disclose intervertebral prostheses, but the prostheses are described as solid bodies.
U.S. Pat. Nos. 4,911,718 and 5,171,281 disclose resilient disc spacers, but no inter-connective or containing planes or like elements are suggested, and sealing the entire unit is not taught.
U.S. Pat. No. 6,156,067 discloses an endoprosthesis having a resilient body formed of one or more materials which may vary in stiffness from a relatively stiff exterior annular gasket portion to a relatively supple central nucleus portion.
U.S. Pat. No. 6,964,686 discloses implantable intervertebral disc replacement prosthesis having a deformable flexure with disc member and lower and upper disc supports communicating with one another to provide support to the disc.
A more recent alternative to spinal fusion is replacement of the damaged disc with a motion preservation device, which includes either a nucleus or total disc replacement. The rationale for the development of the artificial disc is to prevent adjacent segment disease. Artificial disc devices can be broadly divided into two categories, those that replace the nucleus only, leaving the annulus and vertebral body end plates intact and those that involve replacement of the disc and addition of prosthetic end plates. Both strategies are directed at restoration of intervertebral disc function. Prosthetic nuclei are described, for example, in U.S. Pat. Nos. 5,047,055 and 5,192,326. United States Patent application US2002/0183848 also discloses a prosthetic spinal disc nucleus that has a hydrogel core surrounded by a constraining jacket.
There are also several different types of commercially available prosthetic devices for use in the cervical or lumbar segments of the spine designed for total disc replacement. For example, the Prodisc® and the Charite® disc are composites of cobalt chromium end plates with a polyethylene core. The Prodisc® is described in U.S. Pat. No. 5,314,477 and the Charite® disc is described in U.S. Pat. Nos. 5,401,269 and 5,556,431. The Prestige® disc is another type of artificial disc that comprises a metal on metal design with a ball and trough articulation. Another type of artificial disc that is gaining popularity in the cervical spine is the Bryan® disc, described in several United States Patent applications including 2004/0098131; 2004/00544411; and 2002/0128715. The Bryans disc is a composite artificial disc with a low friction, wear resistant, elastic nucleus that articulates with two circular metal plates.
It will be appreciated that prior art attempts at intervertebral endoprosthesis have inherent limitations and have been met with limited success. It will likewise be appreciated that there is a need to overcome the limitations of the prior art and to provide an intervertebral endoprosthesis that avoid the problems inherent in the known prior art.
It is an object of the present invention to provide a vertebral disc endoprosthesis which will perform effectively and efficiently within a patient's spine over a long period of time, and which will not encourage degeneration of or cause damage to adjacent natural disc parts.
It is another object to provide a vertebral disc endoprosthesis which does not require pins or other common mechanical hinge elements, yet which permits natural motion of the prosthetic parts and the adjacent natural anatomy.
It is a related objective to provide a new vertebral disc endoprosthesis surgical procedure which will decrease post-operative recovery time and inhibit post-operative disc, vertebral body and spinal joint degeneration.
Another object of the present invention is to provide an intervertebral disc prosthesis that assists in alleviating the symptoms of degenerative disc disease without sacrificing normal spinal biomechanics.
Yet another object of the present invention is a prosthesis that is easily implanted and mimics both the motion and the stiffness of a normal disc.
It is yet another object to provide a method of installing the endoprosthesis so as to accurately mate the endoprosthesis with a preexisting formed bone surface.
An associated object is to provide an endoprosthesis which will encourage bone attachment to, and growth upon, adjacent outer surfaces of the endoprosthesis.
Yet another object is to provide a vertebral endoprosthesis in which the parts are non-oncogenic.
Still another object is to provide a vertebral disc endoprosthesis having a resilient element to accommodate shocks and other forces applied to the spine.
Another object is to provide a highly effective vertebral endoprosthesis which includes several disc endoprostheses.
A related object is to provide these elements in a pre-assembled array for implantation in a patient.
Another object of the present invention is to provide tools and methods for inserting and positioning spinal disc members into a patient and may also include tools and methods for securing the spinal disc members together.